I should add in Apple's defence that Snow Leopard is a very old outdated OS, the iPhone was in its infancy at that time and an upgrade to 10.7 was expected to be a vast improvement with it's App Store availability instead of the old Software Update and early cloud features, so although many would say it was not as stable it was a foray into a whole new world of possibilities. I don't think it was unfair or unrealistic for Apple to expect people would want to move on especially with the App Store access with free updates and eventual free upgrades. I defend your decision to remain at Snow Leopard but you will appreciate that you are five operating systems behind and the fact that you can still upgrade to the current OS is pretty good. So most of what we can offer are stop gap measures to keep you going until the inevitable demise of your machines which I assume are at least five years old and nearing the end of their expected lifespan.
I think that Iclev's suggestion of using cloud backup on your iPad is a good one. Making it independent of your computers will offer more security for contacts, passwords and media than depending on a local backup to a five year old computer and when you eventually step up to a new machine syncing should be a breeze.
Lastly I should mention that it is possible to backup your iPad to another Mac running iTunes 12.4 if, for example, you have a family member with a Mac running the current or later OS with updated iTunes.
PS please ask us for instructions on this if you decide to try that option.